This invention relates to a magnetic resonance (MR) imaging apparatus for forming an image of a certain cross-section of a body to be inspected by using nuclear magnetic resonance (NMR), and more particularly to an MR imaging apparatus having an improved receiving coil for receiving nuclear magnetic resonance signals.
The MR imaging apparatus usually uses hydrogen nuclei (protons) in a living organ as the detecting target to be inspected. A living body is placed in a static magnetic field Ho. A pulse-like electromagnetic wave is irradiated externally onto the living body to excite protons in the body. When the pulse is cut off, the excited protons return to the original energy state and, upon this transition, emit the once-absorbed energy to the external as electromagnetic waves. These released electromagnetic waves are the NMR signals. A receiving coil for receiving these NMR signals is placed around the body to be inspected to receive the NMR signals. The received signals are amplified and processed to form an image.
In a conventional MR imaging apparatus as shown in FIG. 8 , a body to be inspected or measured (human body) 1 is carried into an accommodation space 19. Then, diagnosis of an abdominal part or a breast part is performed utilizing the nuclear magnetic resonance, and a cross-section 11, e.g. of an abdominal part, of a human body is displayed on a picture plane, as shown in FIG. 7 .
The human body 1 is laid down on a slide plate 12 of a bed 17 and is carried into or out of the accommodation space 19 by the horizontal transport operation of the slide plate 12, as shown in FIG. 8. The accommodation space 19 is defined to be surrounded by a plurality of static magnetic field devices (magnets for establishing a static field) 14 and a plurality of gradient magnetic field devices (coils for generating a gradient field) 15, which devices 14 and 15 all have respective ring shapes. An RF irradiation coil 16 serving as a transmitting coil and a receiving coil 10 for the whole body use, both of circular ring shape, one disposed inside the static field magnets 14 and the gradient magnetic field coils 15. The RF irradiation coil 16 is provided to generate a high frequency magnetic field for causing NMR phenomenon. A unit 20 serves for applying signals to the static magnetic field devices 14, gradient magnetic field devices 15 as well as supplying signals to the RF irradiation coil 16 and receiving signals from the receiving coil 10 and for image-processing the NMR signals detected.
The space inside the receiving coil 10 should be large for giving tolerance, as shown in FIG. 7 , due to the facts that a body 1 to be inspected should be carried into and out of the accommodation space 19 through one end of the receiving coil 10, and that the receiving coil 10 should allow various sizes of the human body to pass through. As a result of this arrangement, the receiving sensitivity of the receiving coil should be low compared to the receiving coil for inspecting a head part.
The receiving sensitivity of a coil, i.e. the S/N ratio of the NMR signal is proportional to the filling factor .eta., ##EQU1## where K is a constant and Q is the quality factor of the coil. The filling factor .eta. is a coefficient representing the ratio of the volume occupied by the body to be inspected in the volume of the coil. A larger filling factor represents that the body to be inspected is disposed closer to the coil.
For the conventional receiving coil for the whole body use, the accommodation space is designed large enough as described above and hence the receiving sensitivity should be low compared to the coil for inspecting a head part.
A surface coil method has been proposed recently, which can solve the above problem.
According to the surface coil method, a coil is directly placed on a portion of the specimen to be imaged. Therefore, it is possible to increase the filling factor compared to the conventional receiving coil for the whole body use and even to the conventional receiving coil for the head part. The detectable region, however, is determined by the coil diameter. The detectable region of a surface coil should be narrow and the detection sensitivity in the depth direction cannot be assured. The detectable region is limited to a region of about a radius of the coil diameter.
For general knowledge of the MR imaging apparatus, reference should be made to, for example, JP-A-61-71103(U), JP-A-61-159949, and JP-A-61-59806.
As described above, in the conventional MR imaging apparatus, the receiving coil for the whole body use is disposed inside the magnets and requires an extra space for accommodating a patient with sufficient tolerance. Thus, the filling factor of the coil was low and the detection sensitivity could not be increased. The surface coil can detect a part of a patient with a high sensitivity, but suffers from a narrow detection range.